Darwin's right: read the coupler, or if the trailer is still with the previous owner have him read it. Many people don't realize the information is there, because it is generally stamped in (not printed on a label).
Since those ball mounts with three permanently mounted balls do not allow adjustment to the correct height (they're zero-rise, for all sizes, only), I would not want one. I vote for Donna's solution: get both, and use the one which is right. Balls are so cheap you could give away the extra one and it wouldn't cost much more than the cost of driving to the store to return it.
1979 Boler B1700RGH, pulled by 2004 Toyota Sienna LE 2WD
Information is good. Lack of information is not so good, but misinformation is much worse. Check facts, and apply common sense liberally.
STATUS: No longer active in forum.